GAZETTE:  You explore some other big myths in the book with anthropological evidence backing you. Let’s talk about a few. Will running eventually wreck your knees?

LIEBERMAN:  That one’s an old chestnut, and it’s just so distressing to hear over and over again. It is true that knee injuries are the most common running injuries. So yes, there’s an association between running injuries and knee problems. But most of the knee injuries that people get from running can be treated and, even better, can be prevented by learning to run properly and adapting your body, especially by strengthening muscles in your hips and legs. I think we could avert a lot of injuries by treating running as a skill. The other problem that people mistakenly worry about is osteoarthritis. Osteoarthritis, unfortunately, is untreatable and a serious problem, but the notion that running causes knee osteoarthritis is not supported by the evidence. There are multiple compelling scientific studies which show that runners are not at greater risk of getting this disease and, if anything, running is slightly protective. Of course, once you do have arthritis, then running can be very painful and can exacerbate it. But let’s dispel the scary myth that running will give you arthritis.

GAZETTE:  Is it normal to exercise less as we age?

LIEBERMAN:  I think this is the most important myth that I bust in the book. We invented the concept of retirement in the modern Western world, and along with that the notion that once we hit 65 it’s normal to take it easy. But that was never true for our ancestors. There was no such thing as retirement in the Stone Age. In fact, until recently, it was the opposite, because hunter-gatherer grandparents often work harder than parents foraging for surplus food that they provide to their children and grandchildren. We evolved to be physically active throughout the entire lifespan. And in turn lifelong activity helps us live longer and stay healthy as we age. This is because physical activity turns on a broad range of repair and maintenance mechanisms that counter the effects of aging. A consequence of this evolutionary legacy is that of all the times to be less physically active, probably the worst is as we get older, because being sedentary deprives us of all those anti-aging mechanisms activated by exercise. Study after study shows that the health benefits of physical activity become more important, not less important, as we age. In fact, the first major study to show this definitively was the Harvard Alumni Study led by Ralph Paffenbarger. That study found that older Harvard alums who were exercising had about 50 percent lower mortality rates than their classmates who were sedentary, and that the benefits of exercise were much higher in older than younger alumni.

GAZETTE:  Is sitting really that bad for us?

LIEBERMAN: This is another example of how the oversimplified way we think and talk about health and physical activity can create confusion. We’ve so demonized being a couch potato that sitting has been called “the new smoking” and it is widely assumed that until recently no one sat very much. But surprise, surprise, when researchers study modern-hunter gatherers, they sit about 10 hours a day, just as much as most Americans. Demonizing something as normal as sitting isn’t very helpful. A much better approach, a more sophisticated approach (which I explore in the book) is to realize that there are better and worse ways to sit. One is to sit more actively. While we evolved to sit a lot, we didn’t evolve to sit motionless for hours on end, and there is compelling evidence that it is helpful to interrupt your sitting on a regular basis. If you are working at a desk, get up every once in a while, fidget, go get yourself a cup of tea, whatever. Those regular, frequent interruptions turn on your muscles and other aspects of your metabolism just enough to lower levels of blood sugar and fat and counteract other negative effects of being sedentary. It’s bit like turning on your car engine. However, it is true that sitting is a problem if that’s all you do. Leisure-time sitting is far more strongly associated with poor health outcomes than work-time sitting. So, if you sit a lot during work but also sit a lot in the mornings, evening, and on weekends, all that sedentism is going to be a problem. All things considered, it’s unhelpful to equate sitting with smoking. Unlike smoking, it’s perfectly normal to sit. It shouldn’t be a source of shame to use a chair and a standing desk isn’t a substitute for exercise. Just don’t spend all your day in a chair. But who doesn’t know that?

Daniel Lieberman will discuss his new book, “Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding,” at a virtual event 7 p.m. Tuesday.  The Harvard Science Book Talks series is a collaboration between the Harvard University Division of Science, Harvard Library, and Harvard Book Store. This event is co-sponsored by The Leakey Foundation.

This interview has been edited and condensed for length and clarity.